Hydrogen generators

ABSTRACT

A pyrolytic hydrogen generator comprising a pressure vessel containing a plurality of cardboard receptacles for the thermally decomposable hydrogen generating material and an associated ignition system. Also, a modular pellet tray assembly for use in the generator comprises a plurality of trays having pellet holders and associated igniters and held in a stack by support rods that also provide electrical connectivity to the trays.

This application is a divisional of U.S. application Ser. No. 12/681,996filed Jun. 11, 2010, which is a 371 of international applicationPCT/GB2008/003475, filed on Oct. 14, 2008, which claims priority toUnited Kingdom patent application serial no. 0720174.2, filed on Oct.16, 2007, United Kingdom patent application serial no. 0720173.4 filedon Oct. 16, 2007, United Kingdom patent application serial no. 0720171.8filed on Oct. 16, 2007 and United Kingdom patent application serial no.0818451.7 filed Oct. 9, 2008, the specifications of each of which areincorporated herein by reference.

FIELD THE INVENTION

This invention relates to improvements in hydrogen generators. Morespecifically, it relates to a pyrolytic hydrogen generator in which amaterial can be thermally decomposed to generate hydrogen. The inventionis particularly suitable for lightweight applications including manportable applications, especially as part of a fuel cell system. Thegenerator can, however, be used in other systems that require hydrogenon demand, such as larger fuel cells, hydrogen engines or gaschromatographs.

BACKGROUND OF THE INVENTION

Hydrogen generating systems fall into two broad classes: the generationof hydrogen from liquid or gaseous hydrocarbons, usually referred to asreformation; and hydrogen generation by the decomposition of hydrogencontaining compounds. The decomposition of hydrogen containing compoundscan be further categorised; firstly, decomposition in the presence ofwater, referred to as hydrolysis, and secondly, decomposition by heat,referred to as pyrolysis or thermolysis.

The thermal decomposition of chemical hydrides such as amine boranes andmetal borohydrides is commonly used as a means for generating hydrogen.Early patents described the decomposition of these compounds to producehydrogen in a ‘one shot’ non-controllable reactor for use with highenergy chemical lasers. Applicant's International Patent ApplicationPublication No. WO 02/18267 describes a pyrolytic hydrogen generator inwhich ignition at the hydrogen generating elements is controlled by anignition control system to allow, for example, for successive orsimultaneous ignition of individual pellets in a controllable and loadresponsive manner.

US2005/0142404 details a variety of pyrolytic gas generation systems inwhich heat generating elements are supported in various arrays.

SUMMARY OF THE INVENTION

The present invention provides in a first aspect a pyrolytic hydrogengenerator comprising a pressure vessel comprising a tray assemblycomprising a plurality of receptacles for receiving thermallydecomposable hydrogen generating material, wherein the tray assemblycomprises cardboard components.

The cardboard components will usually comprise one or more layers ofcardboard forming components that act as a barrier and/or as insulationand/or as a support and/or as a retainer and/or as a separator orsimilar purpose. In a preferred arrangement, the receptacles (orhydrogen generating units) comprise cardboard receptacles (or housings).

Hydrogen generators based on pyrolysis are immediately distinguishablefrom ones based on hydrolysis, due to the very different operatingconditions and components. (For example, those based on hydrolysisnecessarily contain devices such as water tight seals for controllingwater ingress.) In a pyrolytic hydrogen generator, it has been foundpossible to replace the traditional prior art ceramic pellet holderswith a cardboard housing without significant contamination of thehydrogen output, since it has been found that they do not substantiallydecompose or degrade in the hydrogen atmosphere despite the elevatedtemperatures. Their inertness in such an environment therefore permitstheir use to optimise energy densities in portable power systems whereit is critical that the hydrogen generator is as lightweight aspossible. Contrary to established understanding, other support orretaining structures or insulating or separating structures maysimilarly be replaced with cardboard structures. Such structures mayinclude, for example, cardboard retaining rings or discs to holdhydrogen generating pellets in place, cardboard barriers or separatorsto prevent gas or other chemical contamination, cardboard supports forsupporting or positioning or separating electrical components or pelletholders (either cardboard or ceramic), or cardboard insulators forinsulating electrical components or for heat insulation.

The term “cardboard” is to be interpreted broadly to include any stiff,moderately thick paper product, including paper products made fromunbleached craft paper and products known as “rocket paper”. Thegenerator may be in a loaded (i.e. containing hydrogen generatingmaterial) or unloaded condition.

In an alternative first aspect, there is provided a pyrolytic hydrogengenerator comprising a pressure vessel comprising a pellet tray assemblycomprising a plurality of hydrogen generating elements for receivingthermally decomposable hydrogen generating material, wherein the pellettray assembly comprises cardboard components.

In a further alternative first aspect, there is provided a pyrolytichydrogen generator comprising a pressure vessel comprising a pluralityof receptacles for receiving thermally decomposable hydrogen generatingmaterial and an ignition system associated with the receptacles forinitiating decomposition, wherein the receptacles comprise cardboardreceptacles.

References hereinafter to an “ignition system” and “ignitors” formingthat system refer to an initiating system for initiating thermaldecomposition of the hydrogen generating material and thedevices/initiators themselves that initiate this. Hence, an ignitormight comprise a heater coil or any other device that can supply heatstrongly.

The cardboard receptacles may comprise separate respective receptaclesor housings and may be arranged randomly or in an ordered array,preferably in close packed arrangements. Separate housings help avoidproblems of cross-ignition, especially in systems where high heat outputfrom the decomposing material is a problem. However, grid or mesharrangements or similar linked networks of cardboard cells (i.e.integral arrangements where housings share common walls) may be usedinstead of, or indeed, in addition to, separate housings.

Prior to operation, the receptacles may each be loaded with an amount ofa thermally decomposable hydrogen generating material. This may be inthe form of one or more pellets, which may be of any shape or size. Anoptional amount of a heat generating material may also be loaded intoeach housing, which material is capable of being initiated by theignition system and is capable of subsequently decomposing the hydrogengenerating material. The pellet configuration may comprise a first layerof a heat generating mixture capable of being ignited by, and arrangedadjacent to, an ignitor (initiator) of the ignition (initiation) systemand a second layer comprising a hydrogen generating mixture.Alternatively, the first layer may comprise a heat and hydrogengenerating mixture, while the second layer or portion is the mainhydrogen containing compound. Suitable heat generating and/or hydrogengenerating mixtures are described in WO02/18267, the entire disclosureof which is incorporated herein by reference; however, ammonia boraneprovides a particularly energy dense hydrogen source.

The cardboard receptacle may be fixedly or removably attached to asupport tray or printed circuit board inside the pressure vessel. Aplurality of separate or linked housings may be provided on one or moresupport trays. Each housing may comprise a tube or concave holder of anysuitable shape for holding the gas generating material. It may have oneopen end, two opposite open ends or two opposite closed ends although acup or box shape with a closed end adjacent the ignition means and anopen distil end will normally be used. However, the distil end may havea lid and may have a fastener to secure the lid in place, whereexpansion of decomposition products is likely.

The cardboard housing may comprise a cardboard tube. This may haveoptional first and/or second cardboard end walls. The tube may be of anysuitable cross-section, but will usually be of circular or squarecross-section. It may be open or closed. The one or both end cardboardend walls, if present, may be formed integrally, but are preferablyformed separately, and may be fixedly or movably attached to one or bothends of the tube, either across the ends, or if of slightly reduceddiameter, disposed as discs or annular washers inside the tube at ornear either end. A cardboard retainer ring may be insertedtelescopically inside the tube, preferably comprising a much shortertube of slightly smaller diameter, to hold, for example, a pellet inplace below it. This will provide the intimate contact necessary forefficient heat transfer, prevent pellet movement during decompositionwhile permitting expansion of decomposition products.

Each cardboard housing may have a respective ignitor associated with it,and arranged to ignite its contents, once loaded therein, on anindividual basis, either successively or simultaneously to otherignitors. Usually, the hydrogen generator comprises an ignition controlsystem arranged to control the rate of ignition at each respectivehousing, for example, to make the system load responsive. Each ignitoris normally located in the cardboard housing's interior.

The cardboard housings may be mounted directly onto circuit boards,thereby dispensing with additional support trays and saving space andweight. Preferably, the pellet holders are mounted on circuit boards bymeans of pairs of flanges extending downwardly from the ignitors.

Cardboard allows tiny amounts of hydrogen to diffuse slowly through thewalls of the housing which may assist in reduced expansion ofdecomposition products, for example, in the case of ammonia boranedecomposition. However, for heat management reasons, the cardboardhousing may have vents to direct bulk flow of evolved hydrogen quicklyout of the housing towards the walls of the pressure vessel. Forexample, such vents may comprise perforations, slits, slots or any othersuitable holes, notches or gaps specifically intended for ventingpurposes. Those requirements may differ depending on whether thehydrogen generator is intended for high power applications, where thegenerator is quite large and cooling of the hydrogen generating elementsis needed to prevent cross-ignition or, low power applications, wherethe generator is compact and heat needs to be retained in the vicinityof the hydrogen generating elements. Matching or aligned vents, e.g.holes, perforations, slits, notches, etc, may correspondingly beprovided in any adjacent structures such as a tube and/or washers and/orretainer rings also forming part of the housing.

Preferably, the vents are arranged in the final assembled apparatus soas to direct the output gas flow outwardly away from the centre of theapparatus, for example, the centre of each tray. In that case, thegenerator may have a controlled ignition system with a programmed firingsequence that, in use, causes the outermost (rings of) pellets to fireprior to any inwardly disposed (rings of) pellets. Where successiveouter regions (e.g. rings) of pellet holders are present, then in useeach region may be substantially all fired, in successive order,starting with the outermost region and ending with the innermost region.

Thermal insulation may be provided within the pressure vessel to reduceheat transfer from the hydrogen generating elements and hence,cross-ignition. However, the insulating properties of the cardboardhousings may obviate the need for any other thermal insulation in thepressure vessel. Thus, in the case of the loaded and operationalhydrogen generator (where the loaded energy density and firing sequenceis defined), the cardboard housings may be so selected and arranged asto prevent cross-ignition during operation, without any other insulationbeing present in the pressure vessel.

The hydrogen generator may be man portable.

There is further provided portable equipment comprising a hydrogengenerator as described above; such equipment may be in the form of afuel cell system.

In an alternative first aspect, the present invention provides apyrolytic hydrogen generator comprising a pressure vessel containing anarray of pellet holders for supporting pellets of any shape, whetherregular or irregular shape, solid or hollow (e.g. rods, tubes, rings,doughnut shaped masses, powder, granules, segments, etc) of thermallydecomposable hydrogen generating material and an ignition system linkedto the holders, wherein each holder is formed from cardboard. Thehousing is lightweight and inexpensive to produce, allows hydrogendiffusion through its walls, insulates so as to prevent cross-ignition,can be perforated to direct the flow of evolved hydrogen, and does notproduce contaminants. The housing may be directly attached to a circuitboard and may be readily adapted to receive different ignitor devices.

In the second aspect of the present invention, there is provided amodular pellet tray assembly for use in a pyrolytic hydrogen generatorcomprising a plurality of pellet holder trays mounted on top of eachother by means of a plurality of support rods to form a stack, whereineach tray comprises a plurality of pellet holders and associatedignitors for igniting any pellets loaded, in use, into the respectivepellet holders, and wherein the support rods also provide the electricalconnectivity within the stack.

Such an arrangement is lightweight, quick to assemble, and mostimportantly is compact, permitting higher energy densities to beachieved. In particular, it avoids the need for external wires to beattached directly to each tray, or attached along the stack.

References to the trays being mounted “on top of each other” should beliberally construed and do not necessarily require the stack to beupright, since it is intended that the trays could lie in anyorientation providing that they are adjacent to one another so as toform a stack.

Advantageously, the electrical connectivity is directed to operate theignitors.

The support rods may form individual columns of support rods extendingthroughout the length of the stack. Such an arrangement providesstrength and rigidity in the structure. The support rods may be single(one piece) rods that extend the length of the stack, i.e. onerod/column, in which case trays of pellet holders may be dropped ontothe rods to build the assembly. This arrangement is strong, has goodelectrical connectivity and is quick to assemble. However, if anyparticular tray needs attention, it is necessary to dismantle the stackdown to the level of the tray in question. Alternatively, support rodsmay extend the height of one tray only or extend the height of two orthree trays (i.e. multiple trays), in which case interconnectionsbetween the rods need to be provided, but there is then the advantagethat the stack can be of variable length. The rods can either connectdirectly to one another, or conducting connectors can be used to providethe electrical connectivity, the later preferably being provided withinthe tray itself. The support rods can still be aligned to formindividual columns of support rods extending throughout the length ofthe stack (as in the case of the one piece rods).

The assembly needs to be as compact as possible, to maximise energydensities, so will usually be formed from complete trays that include aset of pellet holders and a set of support rods roughly the same lengthas the pellet holders, so there is no dead space; the stack is thenassembled by stacking each complete tray on top of the one below. Inthis highly preferred arrangement, the stack can be assembled merely bystacking each complete tray on top of the one below, without the needfor ancillary support rods or connectors.

The support rods are usually received within conducting bores orchannels located in each tray. The channels or bores may be integrallyformed, for example, as part of a circuit board (that forms the tray ora top layer thereof), or may comprise discrete connectors mountedindividually in the trays, either of which are then connected to theignitors. A plurality of annular pin grips extending through each pellettray may be adapted to receive and grip the support rods.

Pellet holders of the respective trays may be aligned throughout thestack stacked one above another to form columns of pellet holders. Thisallows the trays' wiring to be simplified.

Many wiring arrangements are possible. However, a support rod column(either made of one rod or of set of rods) may be dedicated to providingpower to a particular tray, with x columns powering x respective trays.Similarly, a support rod column may be dedicated to connecting onecolumn of aligned pellet holders to earth, there being y columnsconnecting y respective columns of pellet holders. If both types areprovided, any pellet holder on any tray may be isolated and its firingindividually controlled, by a switch, independently of other pelletholders in the same tray or the same column.

Preferably, the pellet holders are circular, in which case each tray isalso preferably circular. In a preferred embodiment each tray has sixpellet holders arranged symmetrically around a central seventh pelletholder. Pellet holders located around the periphery of the trays mayhave outwardly facing venting perforations. However, further outer ringsmay be provided (e.g. 19 pellet holders/tray), in which case all thepellet holders in the outer rings may have outwardly facing ventingperforations i.e. holes or slits only provided on faces that aredirected away from any inwardly disposed pellet holders. In that case,an ignition control system may arrange for outwardly disposed pelletholders to have their ignitors initiated before those of inwardlydisposed pellet holders.

Advantageously, the pellet holders each comprise cardboard housings,although they may also be ceramic. The cardboard housings may be of theform described above. To eliminate dead space, the pellet holders' upperand lower ends will usually abut pellet trays above and below them.Other cardboard structures as described above may also be provided inthe pellet tray assembly.

In a third aspect of the invention, there is provided a pellet trayassembly for use in a pyrolytic hydrogen generator and comprising atleast one pellet holder tray comprising a plurality of pellet holders,wherein at least some of the more outwardly disposed pellet holderscontain only outwardly facing vents. In use, such an arrangement allowsthe outer pellet holders to be fired first without risk ofcross-ignition of the inner holders' contents.

The phrase “pellet holders” should be construed broadly to cover jacketsor wraps surrounding the pellet holders, since the controlled outwardventing needs to be achieved by providing the selective venting inwhatever outer boundary finally controls the exit of gases from theindividual pellet holders. The vents direct bulk flow of evolvedhydrogen quickly out of the housing towards the walls of the pressurevessel and may comprise perforations, slits, slots or any other suitableholes, notches or gaps specifically intended for venting purposes. Forexample, the pellet holder may have only outwardly positioned notches inits top edge which, in combination with an adjacent lid on top of theholder, provide for selective outward venting.

Preferably no holes or gaps should be present in non-centrally locatedpellet holders that would be large enough to permit fast escape ofhydrogen towards inwardly disposed, closely adjacent pellet holders.Usually vents will only be located in one angular section of the pelletholder subtending a minor angle around an outer direction (i.e.direction corresponding to the position of that particular pellet holderrelative to the centrepoint of the pellet holders positioned on thatlevel or tray).

A hydrogen generator may comprise said pellet tray assembly and anassociated ignition control system (capable of controlling the time offiring of individual respective pellet holders) mounted inside apressure vessel and the pellet tray assembly may comprise a plurality ofpellet trays, each having individual separate pellet holders. Theholders maybe arranged in successive outer rings, which rings may be ofany shape (e.g. square, circular, hexagonal, etc). However, selectiveventing can be employed in any arrangement, whether an ordered or arandom array of holders, including any number of successive outerregions, providing that most or all of the holders in each of thesuccessive outer regions have outer vents and that they are fired, inuse, successively in order in an inward direction starting with theoutermost region.

Preferably, in such a pellet tray assembly at least one pellet holdertray comprises ignitors associated with each of the pellet holders andcontrolled by an ignition control system, which system is adapted toinitiate ignitors associated with the outermost pellet holders prior tothe more inwardly disposed pellet holders. Where there are at least twoouter rings (or regions) of pellet holders, (most or all of) theoutermost ring may be adapted to be initiated first prior to the nextinner ring, and this may continue with each next inner ring in orderbefore the central pellet holder or holders is initiated.

There is further provided a method of operating such a pellet trayassembly in a hydrogen generator with the said partially perforatedpellet holders and ignition system, whereby the ignitors associated withthe outermost pellet holders are initiated prior to the more inwardlydisposed pellet holders. There is further provided portable equipmentcomprising a hydrogen generator as described above, which equipment maybe in the form of a fuel cell.

There is further provided a number of alternative fourth aspects of theinvention which concern the provision of partitioning (or separators)around the hydrogen generating units to provide various technicaladvantages. The features hereinafter described may be combined with anyfeatures described above, except where those clearly conflict. Thus, forexample, the hydrogen generating elements and arrays may be aspreviously described, with the arrays being located on respective traysas previously described, which in turn may form part of assemblies aspreviously described.

Thus, in a fourth aspect there is provided a pyrolytic hydrogengenerator comprising an array of hydrogen generating elements arrangedside-by-side and separated from one another into cells by partitioningprovided with directional venting designed to permit gases to exitlaterally in outward directions only with respect to the centre of thearray.

As a result, the venting of gases sideways in any array, whethercircular, rectangular, hexagonal, or any other suitable shape dependingon the shape of the generator, can be controlled so as to preventcross-activation, which is usually a problem in the centre of suchside-by-side arrays of such elements. Some or all of the elements may beseparated from one another into cells, and usually each cell willcontain only one element.

Usually each element will be operatively linked to a control system,usually each element being linked to a respective ignitor, which systemcontrols the firing of the elements in a selected sequence. Initiationof pyrolysis of the hydrogen generating elements may be controlled by acontrol system in a sequence that minimises or avoids hydrogengenerating elements venting outwardly towards unfired adjacent hydrogengenerating elements. The sequence should be such that when most, or all,of the elements are fired, any elements disposed roughly outwardly fromeach of those elements has already been fired. The outermost hydrogengenerating elements will usually be fired first. This will usually besuccessively followed by progressively inwardly disposed hydrogengenerating elements (although there may be one or two odd exceptions)such that inwardly disposed hydrogen generating elements only fire andvent in the general direction of already fired, outer hydrogengenerating elements.

Preferably, the directional venting directs (laterally) exiting gasesand any exiting waste products towards adjacent, outwardly disposed,empty spaces. (There may also be upward escape of gas—through gaps—butit is the lateral gas movement that needs to be minimised to preventcross-activation.) In the case of inwardly disposed hydrogen generatingelements in the array, it preferably directs exiting gases and anyexiting waste products towards gaps between neighbouring outwardlydisposed hydrogen generating elements.

The hydrogen generating elements may comprise pellet(s) of hydrogengenerating material and the partitioning separating the elements into(primary) cells may comprise individual housings surrounding thepellets(s), which housings have only outwardly facing vents. The ventsmay be located at a specific angle, e.g. plus or minus 5°, or maysubtend a desired minor angle segment e.g. plus or minus 30°.

Ideally, the housings are additionally surrounded by a system of bafflesextending between them and partitioning them into a further set ofindividual cells. Nested cells (i.e. cells within cells) allows moreflexible control of venting, more choice of baffle/housing materials(insulators/conductors) and may provide more heat dissipation, if thebaffles are thermally conductive. The further set of individual cells(i.e. secondary cells) may contain designated empty spaces disposedoutwardly and adjacent to the housings, and, preferably, the outwardlyfacing vents in the housings direct exiting gases and any exiting wasteproducts towards the designated empty spaces.

The baffles may be substantially porous so that most of their surfacearea allows gases to vent freely therethrough. Mesh or other highlyporous structures, if thermally conductive, provide increased cooling.

The baffles may be formed as a one-piece grid arrangement.

In an alternative fourth aspect, there is provided a pyrolytic hydrogengenerator comprising an array of hydrogen generating elements arrangedside-by-side and a system of baffles extending between the adjacenthydrogen generating elements so as to partition them into individualcells, wherein said system comprises gas confining baffle elements andgas venting baffle elements, which elements control the lateraldirection of venting gases within the array.

The gas venting baffle elements may be made of a heat conductivematerial so as to remove some of the heat from the gases quickly and, ifnecessary, both the gas confining and gas venting baffle elements may bemade of a heat conductive material. However, in certain instances, it ispreferable for the gas confining baffle elements to made of a heatinsulating material, especially when used in the manner described belowto prevent lateral movement within a particular region.

The system of baffles may partition the hydrogen generating elementsinto two or more adjacent, successively inwardly disposed regions ofcells, with respect to the centre of the array. The cells in one of saidregions may comprise gas confining baffle elements that prevent hotgases circulating between adjacent cells in said region. The cells inone of said regions may comprise gas venting baffle elements that allowhot gases to vent outwardly from said region of cells, with respect tothe centre of the array. The gas venting baffle elements are preferablysubstantially porous.

Since generators are usually cylindrical pressure vessels, the generatormay have a circular array of hydrogen generating elements partitioned upby the system of baffles into cells, there being a central region of oneor more cells, and one or more successive outwardly disposed annularregions or rings of cells.

The system of baffles may comprise two or more concentric rings ofcircumferentially extending, gas venting baffle elements intersected byradially extending, gas confining baffle elements, so as to form thecells. The outermost, circumferentially extending baffle elements maysurround the outer perimeter of the array of hydrogen generatingelements, and, they may include a protruding portion on each cell ofdead space extending beyond the perimeter; this may present a greatersurface area for heat dissipation and may accommodate waste productswithin the perimeter from the pyrolysis of the hydrogen generatingelements.

The baffle system may comprise one or more continuous rings forming thegas venting elements, as well as short segmented baffles that can bedropped in place inside the rings, forming the gas confining elements.Cells may be formed with double walls formed from overlapping segmentedgas confining baffle elements. The radially extending baffle elementsmay be straight and extend in a single radial direction outwards (andmay join concentric rings of circumferentially extending baffleelements), but are also advantageously formed of segments extending aspart polygons, especially where these demarcate or capture dead volume.

In a further fourth aspect, there is provided a pyrolytic hydrogengenerator comprising an array of hydrogen generating elements arrangedside-by-side and having a system of baffles extending between theadjacent hydrogen generating elements so as to partition them intoindividual cells, wherein said system comprises a one-piece gridarrangement disposed over the array and formed of a collapsiblehoneycomb structure.

The structure may be heat conductive and formed from an extruded orexpanded metallic honeycomb structure of the collapsible concertinatype, or may be formed from extruded plastics, cardboard or otherinsulating materials. The system may be used over a part (e.g. centralportion) or the whole of the array. The hydrogen generating elements arepreferably equally sized and are equally spaced from one another in aregular, preferably hexagonal, array.

It will be appreciated that apparatus in accordance with any one of theabove-mentioned fourth aspects may also contain some or all of thefeatures of one or more of the other aspects; apparatus embodying allthe first three aspects, or all four aspects, provides especially goodperformance.

DESCRIPTION OF THE FIGURES

The various aspects of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a perspective view of an assembled pellet holder for use in ahydrogen generator according to the first aspect of the invention;

FIG. 2 is a perspective view from above of a pellet tray assemblyaccording to the second aspect of the invention;

FIG. 3 is a perspective view of one pellet tray of the assembly of FIG.2;

FIG. 4 is an exploded view of one pellet holder of the tray of FIG. 3;

FIGS. 5A and 5B are perspective views from above and below,respectively, of the circuit board of the tray of FIG. 2;

FIG. 6 is a perspective view from below of the pellet tray assembly ofFIG. 2;

FIGS. 7A and 7B are schematic plan views of two alternative pelletholder arrangements according to the first aspect;

FIGS. 8A, 8B and 8C are schematic sectional views of three alternativesupport rod arrangements according to the second aspect;

FIG. 9 is a perspective view from above of a pellet tray assemblyaccording to the third aspect of the invention;

FIGS. 10A and 10B respectively show two alternative heat baffle systemsfor a 7 pellet holder tray;

FIGS. 11A, 11B, 11C, 11D, 11E and 11F show six alternative heat bafflesystems for a 19 pellet holder tray;

FIGS. 12A and 12B respectively show two alternative heat baffle systemsfor a 19 pellet holder tray and indicate the gas flow directionality;

FIG. 13 is a schematic perspective view of the components of the heatbaffle system of FIG. 12B;

FIG. 14A is a perspective schematic view of a one-piece honeycomb bafflesystem for a 19 pellet holder tray, while FIG. 14B is a schematic viewof part of the honeycomb;

FIG. 15 is a DSC trace comparing the thermal output of a cardboardpellet holder used in a hydrogen generator under oxidising andnon-oxidising conditions;

FIG. 16 is a photograph of the cardboard pellet holders after use in ahydrogen generator under oxidising and non-oxidising conditions; and;

FIGS. 17A and 17B are photographs showing perspective and plan views,respectively, of an alternative baffle system with gas confining and gasventing elements.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a circular cardboard pellet holder for use in ahydrogen source according to the first aspect of the invention isillustrated. The single pellet holder 15 is of an open cup design and isformed from a cardboard tube 16 and a cardboard washer 17, which isfitted inside the cardboard tube 16 at one end to form a removable base.The washer 17 is a tight fit to prevent unwanted heat or hydrogendiffusing underneath to an adjacent cup. (Conveniently the burr from thecutting process can prevent the washer 17 slipping out.) The cup is 2 cmin height, of 1.6 cm internal diameter, with a 1 mm wall thickness.Vents 18 in the form of round holes are optionally provided on oneangular section only of the tube 16 near the top edge, if venting of hotH₂ gas is required. As shown in FIG. 9 below, the holders 15 willnormally be arranged in an array with the vents arranged outwardly fromthe centre of the array to allow venting substantially only in thatdirection.

Any suitable cardboard may be used, providing that it is sufficientlyrigid, has been stored in a dry environment, and is not contaminatedwith undesirable chemical products (i.e. not treated or painted).Unbleached, unprinted cardboard is suitable, and any glues employed inthe lamination of the cardboard or fabrication of components, e.g. tubeforming, should employ adhesives which are inert when heated in thegenerator environment, such as, for example, those based on starch orpolyvinyl alcohol. (Glues which might decompose to give off contaminantsin the output hydrogen gas, for example, products harmful to a PEM fuelcell employed downstream, e.g. cycanoacrylates, should not be employed.)Lightweight cardboard, especially in the range 230-270 g/m², or rocketpaper, may be used for example.

Despite the high temperatures in a pyrolytic hydrogen generator, theinventors have found that cardboard does not char or give rise tocontaminants in the H₂ gas output. In fact, the cardboard does not showany visible degradation in the non-oxidising hydrogen atmosphere whileif the generator is filled with air this causes most of the holder toburn up. This was confirmed in the following tests:—

Pellet Holder (Oxidising Atmosphere)

A single, 3 g (16 mm diameter) pellet of ammonia borane with supportingcardboard cup was positioned centrally on a PCB and then transferredinto a sealed test generator. The pellet was thermally decomposed in thepresence of oxygen by direct electrical heating of the ammonia borane.From the post mortem of the experiment, the cardboard holder was foundto have considerable visible charring.Pellet Holder (Non-Oxidising Atmosphere)The experiment was essentially a repeat of the above—a single, 3 gpellet, 16 mm diameter of ammonia borane was thermally decomposed byconventional means in a sealed test generator although the generator waspurged initially with argon to remove all traces of air and was thenfurther flushed with hydrogen to establish a reducing atmosphere. Afterthe experimental post-mortem, the pellet holder was found to haveretained its structural integrity and no visible charring of thecardboard was observed. FIG. 16 is a photograph showing the cardboardpellet holders after use. (Pellet shown on the right was the one in theoxidising atmosphere.)

None of the decomposition products usually associated with thedecomposition of cardboard (e.g. carbon monoxide, carbon dioxide orcellulose binders) are generated during normal operation. When theoutput gases were characterised by GC-MS and FTIR (Fourier transformInfrared spectroscopy), no peaks associated with such products werefound. For example, detailed analysis using FTIR showed no CO peaks, sothat its concentration was <0.1 ppm (the detection limit of theequipment).

A further experiment compared the stability of the cardboard materialused in oxidising and non-oxidising conditions by the use ofdifferential scanning calorimetry (DSC) and the trace, reproduced asFIG. 15, shows the thermal output recorded. A significant exothermicevent is only recorded in the case of the cardboard used in theoxidising atmosphere.

For portable applications, e.g. portable fuel cells, the lightweightmedium and additional insulating qualities of the cardboard pelletholders (obviating the need for other insulators) allow significantlyhigher energy densities to be achieved in the final apparatus ascompared to apparatus employing tufnol/ceramic cup holders (which canweigh 30% more). Also, as shown, the holders may readily be adapted toallow for venting or other functions.

Where cross-ignition is not a problem, the pellet holders may be linkedin the form of an integral (one-piece) cardboard mesh or grid 32, asshown for example, in FIG. 7B. Cardboard may also be employed asbarriers or supports 33 which may surround (individual or linked) pelletholders of any suitable material. In FIG. 7A, a square support 33provides additional rigidity to individual cardboard pellet holders.

Referring to FIG. 2, this shows a pellet tray assembly containing pelletholders for use in a hydrogen source according to the first, second andthird aspects of the invention. The assembly is intended to be mountedin a cylindrical cartridge (not shown) so as to form the hydrogengenerator, which could be linked, for example, to a fuel cell for use asa portable power source.

Cylindrical pressure vessels are usually preferred, and hence, thepellet tray/assembly is based on circular symmetry. The pellet trayassembly 1 comprises an elongate cylindrical stack 1 of circular pellettrays 2, mounted one on top of another (although it can be operated inany orientation). The assembly is a modular arrangement which can beadapted to meet different load requirements, but in the currentarrangement comprises 6 trays. Each pellet tray 2 comprises a circuitboard 3 on which are mounted seven pellet holders 4. The layout of eachpellet tray is shown in FIG. 3. The pellet holders 4 are arranged in ahexagonal arrangement (between support tubes 5) with the seventh pelletholder placed at the centre. The pellet holders 4 in adjacent trays arevertically aligned so that they occupy identical positions throughoutthe stack 1.

Brass rods or support tubes 5 extend between adjacent trays 2 and arealso aligned vertically throughout the stack 1 so as to form risingcolumns of support tubes 5. The rods are preferably hollow to reduceweight. These support tubes perform a dual function of connecting thestack together and providing electrical connectivity to the pelletholders of the respective trays. Some support rods may be provided toprovide electrical connectivity to other sensors or devices locatedinside the stack 1, for example, thermisters (not shown), while alsoproviding rigidity.

In the current embodiment, there is a set of rods associated with eachtray level, the rods being roughly the same length as the pelletholders. The upper and lower ends of the tubes 5 are held fast inannular conducting pin grips 6 mounted on respective adjacent trays, theupper ends preferably being soldered into the pin grips 6. Referring toFIGS. 5A and 5B, the pin grips 6 extend beyond both faces of each tray,although they mainly project upwardly from the upper faces 7.

A circular dust filter/seal 8 is attached to the lower surface 9 of thecircuit board. As may be seen, a diode 10 is located under eachrespective pellet holder 4 to ensure one-way flow, the diodes and pingrips 6 projecting through the seal 8. The top end of the stack 1 issealed with an end capping circuit board 11 and the bottom end has anadditional stack connection circuit board 12 provided with a connector13.

FIG. 4 shows an exploded view of the components of a single pelletholder 4, when loaded and ready for use. The actual pellet holder 4itself comprises a cardboard support disc 20, a cylindrical cardboardhousing or tube 30 and a cardboard retaining ring 26 inset inside thehousing.

On top of the cardboard support disc 20, there is placed a mica disc 21,a polyimide backing layer 22, and an ignitor/initiator in the form of aheater coil 23. Next, the housing 30 is loaded with the renewablechemical components, namely, a heat mix pellet 24 and a hydrogengenerating pellet, in this case, a pressed ammonia borane pellet 25. Thelatter is held in place by the cardboard retaining ring 26, which is atight fit inside the tube 30. An aluminium paper lid 27, an aluminiumfoil lid 28, and a glass wool plug 29, are placed in turn above thepellet for insulation and to prevent significant upward flow of H₂ gasto the tray above.

Each pellet holder 4 is pinned to the circuit board 3 by virtue of apair of downwardly extending conducting flanges 14 that form the twoends of the heater coil (as shown in FIG. 3). Each of the 6 outer pelletholders 4 is partially perforated (according to the third aspect) on theportion of the tube 30 that faces outwardly, the perforations 31directing the hot generated H₂ gas away from the other pellet holders 4to prevent cross-ignition; the retaining ring 26 disposed inside thetube 30 has correspondingly aligned matching perforations. The centralpellet holder 4 is surrounded by other holders, and is fired last; ithas no perforations and is therefore protected from lateral gas flow.

The stack 1 is assembled by first assembling the stack connectioncircuit board 12 and then the individual pellet trays 4. As regards thelatter, perforations 31 in the cardboard housing 30 need to be facingoutwards and the components inside are compacted to ensure good contact.In particular, the heat mix pellet 24 needs to be in a closely abuttingrelationship with the ignitor coil 23 and hydrogen generating pellet 25.

The lowermost pellet tray 4 is placed upon the stack connection circuitboard 12 using a shorter length set of support rods 5. Next therespective individual “complete” tray modules 4 are added. In thisparticular arrangement, the (longer) standard length support rods 5 arepermanently soldered to the underside of the pin grips 6 so as to extenddownwardly from a particular tray (see FIG. 8B) and are removablymounted onto the pellet tray 4 immediately below, from which tray a setof pellet holders extend upwardly (akin to stalactites and stalagmites).A “complete” tray module is indicated by bracket A on FIG. 2.Thereafter, the required number of “complete” pellet tray modules aresuccessively added to the stack and compressed to ensure a rigid stack.The end capping circuit board 11 is then fixed in place at the top end.

The stack 1, when loaded with pellets, can be inserted top end firstinto any suitable cartridge hydrogen generating pressure vessel, such asfor example, a cartridge, as is known to the person skilled in the art(and described, for example in WO02/18267). The stack connection circuitboard 12 will be connected to a suitable interface/end cap sealing thepressure vessel.

In the current arrangement, the circular trays are 5.6 cm diameter, withthe circular pellet holders of ˜1.6 cm diameter and ˜2 cm height. Eachpellet holder can hold 1 g of ammonia borane so that a 7 pellet tray iscapable of holding 42 g of ammonia borane and generating 74 liters of H₂gas. A stack of, for example, the current size would be suitable forpowering a 50 W fuel cell for 2 hours, and hence, would be suitable forpowering a 50 W device. e.g. a portable TV camera or radio equipment.

A preferred firing sequence would leave the firing of the central pelletin each tray to last, the outer pellets in each tray being fired first.It is also preferable to stagger the pellet firings around the stack(e.g. alternate opposite sides) to avoid the firing of adjacent pelletson the same tray or of adjacent pellets closely located in trays aboveor below each other, to avoid hot spots developing.

If desired an additional larger ring of pellet holders could be arrangedoutside the six pellet holders on a larger tray, providing that thepellet holders again have only outwardly directed perforations. Such a19 pellet tray 19 is shown in FIG. 9. As long as the firing sequenceensures that the outermost ring of pellets is fully or nearly fullyfired before the next innermost ring of pellets is initiated, it ispossible to avoid cross-ignition of the more centrally located unfiredpellets. Such an arrangement and method is in accordance with the thirdaspect of the invention.

Example

In a test demonstration, the stack was loaded with 42×1 g pellets ofammonia borane (AB) and inserted into an experimental cartridge sealedby a purpose-designed interface control system to form a test hydrogengenerator. This was linked to a 100 W PEM fuel cell. The cartridge wasflushed with argon, and then pressurised with H₂. The total systemweight of the AB fuel, cartridge and fuel cell was 3.47 kg.

The test lasted 51 minutes with the control system tripping outprematurely after 42 minutes so that only 32 of 47 pellets wereactivated. The measured average voltage was ˜13V, the measured averagecurrent was ˜3.5 A and the calculated mean power was ˜46 W, which wouldintegrate to a total energy of 58 Wh for this set-up. The mean powerdensity (for the 3.47 Kg system) was 13 W/Kg.

In a further test, the same hydrogen generator was re-loaded and theload was increased to ˜7.5 A, ˜11V to deliver a peak power output of ˜80W, which was easily sustained for a few minutes. While pyrolytichydrogen generators, as solid state systems, are intrinsically robust,the current stack arrangement has been found to be especially robust andreliable. In a further test, the system was successfully operatedupwards, downwards, and on both sides (i.e. in 4 orientations passingthrough 360°) and was found to exhibit dynamic orientation independence.

It will be apparent to the skilled person that numerous modificationscould be made to the current arrangement, still in accordance with thevarious above aspects of the present invention. For example, while it ishighly preferred for the support rods 5 to occupy identical positionsthroughout the stack 1, so that direct connections can be made betweenrods 5 (and pin grips 6) from neighbouring trays, and structuralrigidity can be optimised, it would be possible to use otherarrangements where, for example, electrical connectivity has to beprovided to non-aligned support rods of neighbouring trays via thecircuit board on the trays. This would mean that adjacent trays wouldhave different arrangements, for example, two different types of circuitboards in an alternating arrangement of the two types of trays, and mayrequire the provision of additional pin grips that would only provideeither an upward or a downward electrical connection, rather than bothtypes of connection as in the present arrangement.

While the current pellet tray arrangement with round pellet holders andround trays is preferred, other holder/tray arrangements are alsopossible.

The current individual sets of support rods allow ready dismantling ofthe stack and addition/removal of trays. As shown in FIG. 8B, each(short) support rod 5 is fixed by a soldered joint 34 into the base ofthe pin grip 6 on the underside of the PCB 3 and extends down to the pingrip 6 of the tray below. However, assembling each tray module is timeconsuming and the soldered joints 34 may fail. Hence, the individualsets of short support rods 5 may be replaced with a single set ofsupport rods 5′ the same length as the final size of the stack, wherethat size is unlikely to change. Such rods are shown in FIGS. 8A and 8C.In all of FIGS. 8A, 8B and 8C, the pin grips 6 pass through and protrudefrom both the upper and lower sides of the PCB 3 (although the lowersection is not shown), allowing use of either the long 5′ or short 5rods therein. In FIG. 8A, the respective trays 3 are linked together byindividually manually inserting the full stack length rods 5′ (150 mm)freely through a line of pin grips 6, and thereafter secured at each endafter all of the rods 5′ have been inserted. In FIG. 8C, the full stacklength rods 5′ are soldered on to a top plate PCB 35 at one end, witheach tray 3 then being slid onto the stack 1 from the open end andpushed up tight against its neighbour before the connector PCB 36bearing a 20 way connector 37 is attached.

Various heat baffle systems in accordance with the fourth aspect of theinvention and intended to assist in reducing cross-activation ofhydrogen generating elements will now be described with reference toFIGS. 10 to 17. These systems may all be employed in a pyrolytichydrogen generator such as, for example, described above according toany one or more of the other inventive aspects, and which is loaded, inuse, with hydrogen generating material. The systems are, however,especially applicable to a hydrogen generator based on ammonia boranethermal decomposition, and particularly ones where there is a heatmanagement requirement, for example, where there is a high productionrate and/or larger storage capacity. For example, management of excessheat is likely to be necessary in fuel cell applications with powerrequirements of 15 W or more, or in non-fuel cell applications where ahydrogen production rate averaging around 0.1-0.2 slpm (standard litersper minute) or more is required.

Referring first to FIGS. 10A and 10B, these show two alternative heatbaffle systems 40, 42, both one-piece grid arrangements with seven cellsdesigned to fit around an array of seven cardboard pellet holders 44 ona tray (not shown), such as for example, shown in FIG. 3 above, andcontaining ammonia borane pellets. In both cases, they would be usedwith cardboard pellet holders 44 having only outwardly facing holes orslits. These could be spread over a narrow angle (e.g. up to 50°), asshown in the seven pellet trays depicted in FIG. 2 above, or could bearranged vertically above one another and located at a specific angle,as discussed in relation to FIGS. 11B and 11C below. The nested cellsconfiguration (cell defined by a housing within a cell defined by baffleelements) provides flexibility in how gas flow is controlled.

In each one-piece baffle system, a central hub 46 surrounds the centrepellet holder with vanes 48 extending radially and symmetricallyoutwards connecting to an outer ring 50. In each system, the walls ofthe hub and vanes are non-porous (i.e. have no through openings) whilethe walls of the outer ring are porous (i.e have throughopenings—preferably of at least 1 mm width); by “porous” is includedperforations, slits, slots, mesh or any other suitable holes, notches,etc. The outermost pellets would be fired before the central pellet. Thenon-porous walls substantially prevent the passage of hot gas orparticulates laterally (sideways) from any one cell to adjacent cells inthe outer ring or inwardly towards the central pellet holder. Thisreduces the potential of cross-activation which could result in lostefficiency or, indeed, thermal runaway of neighbouring pellets. Theporous outer ring walls, which may for example be perforated, slotted,slitted, made of mesh, or similarly porous, in combination with theoutwardly venting pellet holders, allow hot gases to vent outwardly awayfrom the pellets per se.

In FIG. 10A, the spokes connect to a circular outer ring baffle element50, while in FIG. 10B, this is replaced by a star-shaped perimeterbaffle element 50′. The latter is preferred in that it has a highersurface area and has dead volume 52 within the cells that canaccommodate residue products. (In the case of ammonia borane, thefoaming residue products from the decomposed pellet can cause problems.)

The baffle system is formed of a (preferably lightweight) conductivematerial, which is non-reactive in the hydrogen generator atmosphere andusually metallic; examples include aluminium, copper, or titanium ortheir alloys. In this case, aluminium foil hubs and vanes and aluminiummesh outer rings were used. The metallic spokes, hubs and outer baffleelements all serve to conduct away the heat from a pellet that is firingand dissipate that heat over a much larger area, hence minimisinghotspots. The use of a mesh outer ring is especially preferred since thehigh surface area of the mesh is particularly efficient in cooling thegases passing through its openings, as well as directing gasesoutwardly.

Both systems may be the same height or taller than the pellet holdersand may be sufficiently rigid to support layers of pellets in the stackand provide the correct separation between layers when undercompression. The baffle elements also prevent any sideways movements ofthe holders in their relative positions, which is particularlyadvantageous when the holders are not otherwise secured to the tray.

FIGS. 17A and 17B are photographs showing perspective and plan views,respectively, of an alternative baffle system with gas confining and gasventing baffle elements. While the baffle system of FIG. 10 was formedfrom heat conductive gas confining and gas venting baffle elements, inthis baffle system the gas confining baffle elements are formed ofcardboard and installed inside the circular mesh outer gas ventingbaffle element 40, as used in the FIG. 10A embodiment.

FIGS. 11A to 11F show a variety of alternative heat baffle systems for a19 pellet holder tray on which a central pellet holder is surrounded byan outer ring of six pellet holders 54 of the same size, which ring issurrounded by a successive outer ring of twelve larger pellet holders56. FIGS. 11B and 11C depicts the same baffle systems as in FIGS. 10Aand 10B, respectively, but now installed in a 19 pellet arrangement.

It has been found advantageous to use progressively larger pellets insuccessive outer rings of a multi-ring array. For example, in a 19pellet tray with two concentric rings, regardless of the baffle system,if same size pellets are used throughout and the pellets in the centreare too large, the heat output will trigger cross-activations. However,such larger pellet sizes can be tolerated in the outer rings where thepellets are spaced further apart, allowing packing density to bemaximised without cross-activation. Thus, in this case, the use of 19 mmpellets throughout the tray triggered cross-activations in the centre,while 16 mm pellets throughout the tray were acceptable. It was,however, found that the use of 18 mm pellets in the outer ring alongsidethe central 16 mm pellets was also acceptable, and maximised packingdensity.

The pellet holders are all provided with a vertical column of ventingholes 70 (similar to those shown in FIG. 14A) that provide directionalventing at only a specific angle 72 and the venting holes are alwaysdirected towards dead (empty), outwardly disposed spaces 74. Moreover,the venting holes in the inwardly disposed pellet holders are alwayslocated at the gaps 76 between pairs of adjacent pellets holders in thenext outer ring.

In each figure, the direction of venting of every pellet holder isindicated by markings 72. In every arrangement, the baffle system is aone-piece grid arrangement. The baffle elements in the system may all benon-porous (as hereinbefore defined) hence preventing lateral ventingand causing gas to vent vertically, or may all be substantially porous,in which case venting tends to follow the directionality in-built in thepellet holders although gas moves in other lateral directions as well.However, preferably, all the radially outwardly extending baffleelements are non-porous i.e. solid walled with no through openings,while the outer ring shaped baffle elements are all substantiallyporous.

Since the inner ring of pellets are more densely packed and moresusceptible to cross-activation it may only be necessary and desirable(for weight reasons) to use a baffle system around the inner rings ofpellets. However, the FIGS. 11 11E and 11F arrangements provide thebaffle systems with the largest thermal mass and hence ability todelocalise and dissipate heat so as to prevent cross-activation.

Example

Experiments were conducted on the FIG. 11B pellet tray arrangement, withand without the circular baffle design 50. The board incorporated 18 mmpellets around the circumference and an internal ring of 16 mm internalpellets and a single central pellet. For both experiments the testgenerator was used with a backfill of hydrogen (10 bar) and conditionedto 40° C. to simulate the higher storage/working temperature of thegenerator. The initiation sequence consisted of decomposing 18 mmpellets around the circumference and then initiating the inner layer ofpellets.

Cross activation of the inner pellets occurs as a result of (i)convection of hot gases from pellets decomposed from the outer ring, and(ii) conduction of heat from initiating the inner ring of pellets due tothe closer proximity of the pellet positions compared to the outer ring.

The aluminium mesh baffle 50 addresses both cross activation scenariosby diffusing/cooling hot gases from the outer ring (a 50° C. drop intemperature across the baffle has been observed) and the solid separator(spokes) acts as a physical barrier to lateral gas flow. The use of thebaffle resulted in no cross activation of the inner ring of pelletscompared to total of 28% of internal pellets cross activated without thebaffle (or 18% of pellets cross-activating in total, considering allpellets on the tray).

FIGS. 12A and 12B show two further heat baffle systems for a 19 pelletholder tray. The trays again comprise a central pellet holder and twoconcentric outer rings of pellet holders, the outermost ring containinglarger pellet holders, with a baffle system only located around thecentral pellet holder and next adjacent outwardly disposed ring. Thesesystems are quick and easy to assemble and involve the use of acircumferentially extending baffle element 80 comprising a mesh ring,and segmented, half-hexagonal, solid walled baffle elements 82, whichare all dropped into place. This is shown in FIG. 13, which is aschematic perspective view of the components of the heat baffle systemof FIG. 12B.

The pellet holders are merely numbered for ease of identification; thenumbering does not indicate their firing sequence.

Once again, the pellet holders are all provided with a vertical columnof venting holes (similar to those shown in FIG. 14A) that providedirectional venting at only a specific angle, as depicted by the radii84 extending out from the centre of each pellet holder. In botharrangements, the venting holes are always directed towards dead (empty)spaces 86. Moreover, the venting holes in the inwardly disposed pelletholders are always located at the gaps 88 between pairs of adjacentpellets holders in the next outer ring. Thus, in FIG. 12A, inner pelletholder No. 17 is seen to vent towards the empty space between outwardlydisposed pellet holders No. 10 and No. 11. The latter outermost pelletholders vent towards outwardly disposed dead spaces as well.

In FIG. 12A, the segmented baffle elements are arranged symmetrically,with radially outwardly extending limbs. There is the advantage that twothicknesses of baffle elements lie alongside one another betweenadjacent cells in the inner ring. However, any foaming residues are alsoexpelled directly onto the mesh baffle element, which can cause blockingof the mesh. In FIG. 12B, the half hexagons are oriented differently togenerate asymmetric dead areas 90 within the cell (adjacent the gaps inthe outer pellet holders) and the venting of the inner ring of pelletholders is staggered (rather than purely radial) so that venting isaimed at the dead spaces, thereby minimising blocking of the mesh baffleelement. Thus, in FIG. 12B, pellet holder No. 17 is seen to vent towardsthe empty space 88 between outwardly disposed pellet holders No. 11 andNo. 12.

In a typical firing sequence, the outer pellets are generally firedbefore inner pellets, although an inner pellet can be fired as soon asthe outwardly disposed, adjacent pair of pellets have fired. Hence, inFIG. 12A, once pellet holders No. 10 and No. 11 have been fired, pelletNo. 17 can be fired, as it will only vent onto already fired pellets.

FIGS. 14A and 14B show an alternative heat baffle system for a 19 pelletholder tray similar to that of FIG. 9 above. FIG. 14A is a perspectiveview of the whole tray, but showing schematically only a portion 92 ofthe baffle system, while FIG. 14B shows three adjacent cells that wouldbe formed by the baffle system, each containing a circular pelletholder. In this tray, the pellet holders are hexagonally close packedand are all the same size. They are separated by a hexagonal,honeycomb-type, ready-made baffle system constructed from extrudedmetal, such as, for example, extruded (expanded) aluminium. Suchmetallic honeycomb structures are available in a variety of cell sizesand cell heights. They are lightweight and flexible and quick and easyto install since they collapse/expand concertina fashion and may be usedover a part (e.g. central portion) or the whole of the array. They mayalso provide stiffness and structural support to a stack of pellettrays, thereby preventing deflection of the stack under shock and/orvibration. The honeycomb cell height may be selected to provide thecorrect separation between adjacent pellet trays in the stack. Themetallic baffle system again provides a heat sink to cool gas exiting apellet and delocalise that heat, and where the honeycomb has solidwalls, physically obstructs lateral hot gas plumes from cross activatingpellets out of sequence. Collapsible honeycomb structures made ofinsulating material are also commercially available and may be used.

While the honeycomb will usually be solid (non-porous) walled tominimise lateral gas circulation, it may be perforated, slitted orslotted, etc. In this embodiment, the honeycomb was perforated and thedirectionality of the gas flow was controlled solely by the outwardlyventing vents in the pellet holders which, in this case, were a column70 of holes aligned vertically above one another at only a specificangle/point on the holder circumference. It will be seen that the ventson inwardly disposed pellet holders are aligned so as to vent towardsthe gap between adjacent, neighbouring pairs of outwardly disposedpellet holders.

It will be apparent to the skilled person that numerous modificationscould be made to the current arrangement, still in accordance with thevarious above aspects of the present invention.

The invention claimed is:
 1. A modular pellet tray assembly for use in apyrolytic hydrogen generator comprising a plurality of pellet holdertrays mounted on top of each other by means of a plurality of supportrods to form a stack, wherein each tray comprises a plurality of pelletholders and associated ignitors for igniting any pellets loaded, in use,into the respective pellet holders, and wherein the support rods alsoprovide electrical connectivity within the stack.
 2. An assemblyaccording to claim 1, wherein the support rods provide the electricalconnectivity to operate the ignitors.
 3. An assembly according to claim1, wherein the support rods form columns of support rods extendingthroughout the length of the stack.
 4. An assembly according to claim 3,wherein each column is formed from a single support rod roughly the samelength as the stack.
 5. An assembly according to claim 1, wherein theassembly is formed from complete trays that include a set of pelletholders and a set of support rods that are roughly the same length asthe pellet holders whereby the stack has been assembled by stacking eachcomplete tray on top of the one below.
 6. An assembly according to claim1, wherein the support rods are received within conducting bores orchannels located in each tray.
 7. An assembly according to claim 1,wherein pellet holders of the respective trays are aligned throughoutthe stack to form columns of pellet holders.
 8. An assembly according toclaim 1, wherein the pellet holders each comprise a cardboard housing.9. A pyrolytic hydrogen generator comprising a pellet tray assemblyaccording to claim
 1. 10. Portable equipment comprising a pyrolytichydrogen generator according to claim 9.